This invention relates to magnetic resonance (MR) apparatus. The invention is especially concerned with the reduction of artefacts caused by so-called xe2x80x9caliasingxe2x80x9d in magnetic resonance images in magnetic resonance imaging apparatus, and the reduction of artefacts caused by aliasing in magnetic resonance spectroscopy apparatus.
Magnetic resonance apparatus comprises a magnet for producing a main magnetic field to align magnetic resonant active nuclei such as hydrogen in tissue in a patient, and radio frequency excitation means such as a radio frequency coil to excite these nuclei to resonance. The resulting relaxation signals generated by the nuclei are picked up by a receive coil or coils, which may be separate from the transmit coil or may be one and the same coil(s). The signals provide information on the distribution of the nuclei and hence information about the tissue itself. Spatial information on the distribution of the nuclei, in magnetic resonance imaging, is obtained by spatially encoding the MR signal by means for generating magnetic field gradients.
In any magnetic resonance imaging apparatus, the region selected for imaging is within the region of good field produced by the magnetic system. The requirements for the region of good field is that the field of the main magnet must be homogeneous to a very high degree, and the magnetic field gradients imposed on the main field must be uniform to a very high degree.
Thus, referring to FIGS. 1 and 2, in one design of magnetic resonance imaging (MRI) apparatus, the magnet 1 receives patients on a couch 2 into a cylindrical bore 3, and the region of good field extends between radial planes A and B.
In the case of a radial slice being imaged, the particular slice which is selected is determined by the frequency of the radio frequency excitation pulse, which excites nuclei precessing at a certain frequency. The frequency at which the nuclei precess is dependent on the magnetic field strength. For example, in FIG. 1, the axial gradient results in the magnetic field strength increasing (in a linear manner) in an axial direction from A to B. Unfortunately, where the gradient field and the main magnetic field strength fall off, the resultant magnetic field will pass through the same values as over the linear region. Thus, the region D is subject to the same magnetic field strength as the slice B. When slice B is excited, region D will also be excited, and a certain amount of the MR signal generated in region D will be picked up by the radio frequency coil 4 and be processed as if it had been generated by the selected slice B.
This is the problem referred to as aliasing, and is responsible for generating artefacts in the image produced by the MRI apparatus. This is a particular problem with shorter magnets, which have been proposed to reduce problems of claustrophobia, because the fall off now takes place closer to the region of good field than hitherto. Hence, it is more difficult to arrange that no part of the patient lies in the alias region during imaging.
Various proposals have been made to alleviate this problem of aliasing. Thus, it has been proposed to shield alias regions such as D in FIG. 2 from the excitation pulse generated by the radio frequency coil 4, but this requires the use of a separate shield. It has been proposed (British patent application no. 98 11445.7) to use an array of transmitting coils to generate a radio frequency excitation pulse which collapses in the region D.
Arrays of coils have been used before as transmitting or receive radio frequency coils and, it has been proposed (British patent application no. 98 28428.4) to use an array of receive coils in the region of good field (from A to B) in order to reduce the number of slice encode steps which would otherwise be needed to distinguish between different simultaneously excited radial slices.
The invention provides magnetic resonance apparatus, comprising means for producing a main magnetic field and a magnetic field gradient in a desired signal region, radio frequency means for exciting magnetic resonance in a volume of the desired signal region, a primary radio frequency receive coil for receiving MR signals from the desired signal region, an additional radio frequency receive coil for receiving MR signals from an alias region outside the desired excited volume, in which alias region the magnetic field has a value equal to that in the desired excited volume, and processing means for producing data from the desired excited volume which uses signals from the additional radio frequency receive coil as well as from the primary radio frequency receive coil in order to reduce artefacts caused by aliasing in the data produced from the desired excited volume.
The invention takes a different approach to that of shielding or nulling out the radio frequency signal in that a radio frequency coil is provided for collecting the alias signal, and the signal is then used to reduce the effect of aliasing on the desired data.
The present invention provides the foregoing and other features hereinafter described and particularly pointed out in the claims. The following description and accompanying drawings set forth certain illustrative embodiments of the invention. It is to be appreciated that different embodiments of the invention may take form in various components and arrangements of components. These described embodiments being indicative of but a few of the various ways in which the principles of the invention may be employed. The drawings are only for the purpose of illustrating a preferred embodiment and are not to be construed as limiting the invention.